This invention relates to an amplitude modulator, and, more particularly, to an amplitude modulator adapted for integration.
An amplitude modulator has been provided which modulates the amplitude of an amplified output signal by supplying a modulating signal to the base or collector terminal of a transistor. Difficulties have appeared in that a modulating signal leaks to the amplified output from the transistor, giving rise to fluctuations in the reference level (zero level) of the amplified output. To avoid this undesirable possibility, an output signal from the transistors has been drawn out through a transformer. Thus, the conventional amplitude modulator involving a transformer is not adapted for integration. Further, such a conventional amplitude modulator which produces a modulated signal by utilizing the diode characteristic between the base and emitter of a transistor fails to indicate a high linearity of modulation factor with respect to an amplitude of the modulating signal.
An amplitude modulator using a double balanced analog multiplier may be cited as one type which does not employ a transformer, as disclosed in "Applications of a Monolithic Analog Multiplier" by Alberto Bilotti (FIG. 2), IEEE Journal of Solid-State Circuits, December, 1968, pp. 373 to 380. This proposed double balanced analog multiplier has three differential pairs of transistors. The common emitters of the first and second differential pairs are respectively connected to the collectors of the third differential pair of transistors. The first and second pairs are supplied with a first differential input, and the third pair is supplied with a second differential input. The collectors of the first pair are connected respectively to the collectors of the second pair performing the opposite phase operation, and also to the output terminal.
With the above-mentioned circuit arrangement, a product of the first and second inputs appears at the output terminal. Namely, this circuit acts as a double side band (DSB) type amplitude modulator. With the DSB system, a carrier component does not appear, making it necessary to supply a carrier on the demodulator. Therefore, the DSB system is not adapted for general applications.